Actinomyces naeslundii is a resident of the normal oral flora, however, it also has the potential to cause disease, namely root caries and periodontitis (3, 4). Colonization by A. naeslundii can begin during infancy; it can adhere to salivary proline rich proteins on a tooth surface via type 1 fimbriae and/or to surrounding Streptococcus species such as S. oralis via the type 2 fimbriae. In addition, adherence by type 2 fimbriae is associated with lactose sensitive receptors in the host, such as those on mucosal epithelial cells, erythrocytes, and polymorphonuclear leukocytes (55, 57). Since little is known about the fimbriae associated adherence mechanisms in gram-positive bacteria, studies ofA. naeslundii type 1 and type 2 fimbriae make this organism an ideal model to investigate adherence to host cells as well as aggregation to surrounding bacterial cells in a biofilm. This investigation will examine the effects of environmental signals on the synthesis of type 1 and type 2 fimbriae to test the hypothesis that environmental factors can regulate gene expression of fimbriae. The focus of this proposal will include: Aim 1. Determination of environmental signals that regulate fimbrial biosynthesis Real-time reverse transcriptase PCR will be used to monitor the expression of the type 1 and type 2 fimbrial genes ofA. naeslundii while growing in different conditions in a continuous culture system. Aim 2. Characterization of the type 2 fimbrial promoter The transcriptional start site for the type 2 fimbrial locus will be mapped by primer extension and S 1 nuclease experiments. The type 2 fimbrial promoter regions will then be cloned into a reporter gene vector for use, in future grant periods, in promoter mutation studies.